The present invention relates to the art of assembling electronic devices, and more particularly, to an improved non-ionic, water washable soldering paste that eliminates ionic residues and optionally permits printed circuit boards to be washed with water.
Soldering pastes have been used for a number of years in the assembly of electronic components, such as integrated circuit packages which are mounted on a base member, (i.e. a dielectric substrate) and electrically connected to a conductive circuit formed on or otherwise attached to the base member. The soldering paste is applied to selected regions of the conductive circuit by a deposition process, such as stenciling or screening. Electronic components are then positioned on the base member and the entire assembly is fired in a heating chamber to bond the components to the conductive circuit.
Soldering pastes contain a flux that functions to eliminate oxide contamination and/or prevent further oxidation of the solder upon reflow. Reflow is defined herein as the fusion of solder particles into a homogeneous bead of molten solder. Reflow takes place when solder is heated to a temperature equal to or greater than its melting point. Organic components in soldering pastes and fluxes are desirably vaporized during the soldering process, however some components leave residues that must be removed by organic solvents, specifically chlorofluorocarbons. The use of chlorofluorocarbons is extremely worrisome since these materials are reportedly the agents responsible for destroying the ozone layer. Depletion of the ozone layer contributes to increased UV exposure which is the primary cause of skin cancer.
Attempts have been made in the past to develop soldering pastes and fluxes that leave no residue or leave a residue that can be removed with water. These attempts, however, have not been totally successful for various reasons. For example, although commercially available water washable soldering pastes provide a distinct advantage over pastes that leave residues requiring removal by chlorofluorocarbons, the former may leave ionic residues which cannot be totally removed with water. When such residues remain on a circuit board, they create an ionic environment which causes the board to short out.
U.S. Pat. No. 3,223,561, granted Dec. 14, 1965 to Durham et.al., discloses the use of an amine salt in a soldering flux. Although use of an amine salt eliminates the damage that occurs with strong fluxing agents, the amine salt leaves ionic residues which are unacceptable.
In view of the foregoing, it is obvious that a non-ionic, water washable soldering paste would be very desirable.